Solid-state thin film batteries are typically formed by stacking thin films on a substrate in such a way that the films cooperate to generate a voltage. The thin films typically include current collectors, a positive cathode, a negative anode, a solid electrolyte (film) and different protective layers.
In a thin film battery configuration, the cathode layer is separated from the anode layer by an electronically insulating layer of solid electrolyte material. This electrolyte layer provides two functions. The first function is to conduct the electrochemically active ions between the cathode and the anode. The second function is to prevent the direct exchange of electrons between the cathode and the anode so that the electronic current becomes available only in the external circuit of the battery.
The solid electrolyte film and the electrode can be deposited onto the substrate by, for example, sputtering techniques such as radio frequency sputtering (RFS or RF sputtering) and RF magnetron sputtering (RFMS). These techniques are strictly physical vapor deposition (PVD) processes where charged ions of the plasma are bombarded on the surface of the target. Particles are ejected from the target and form a solid thin film layer on the substrate.
The present invention belongs to the field of solid electrolyte film deposition by PVD using cathodic sputtering targets.
Common solid electrolytes that are contained in solid-state thin film batteries must be free from any electronically conductive element, in particular they must be carbon-free, because they must prevent the direct exchange of electrons between the cathode and the anode to avoid any self discharge of the battery. They are often made from lithium in combination with one or several other elements such as phosphorus, silicium, sulphur, oxygen, boron, vanadium, etc. . . . . As examples of lithium-based electrolytes used in the eighties, one can mention the following compositions: Li3.6Si0.6P0.4O4, 0.31 Li2SO4-0.31 Li2O-0.38 B2O3, 0.82 Li2O-0.08 V2O5-0.10 SiO2, 0.55 LiI-0.36 Li3PO4-0.99 P2S5. The electrolytes now generally used in microbatteries since the beginning of the nineties are based on LiPON and are prepared by sputtering starting from a target of Li3PO4 in a nitrogen atmosphere. See, for instance, U.S. Pat. No. 5,569,520, Apr. 30, 1996 and U.S. Pat. No. 5,597,660, Jan. 28, 1997, issued to John B. Bates et al. The formula of LiPON is Li3.3PO3.9N1.7 but can change slightly following the condition of its preparation with a slight change of its ionic conductivity.
Targets used during the sputtering processes used to make thin film solid electrolytes can be manufactured by admixing and reacting together the different components of the target, before melting them to obtain a glass. This glass is however not resistant enough to be used as such in a sputtering deposition process (the glass breaks or crazes) and has to be further grinded, pressed in a mold (for example in the form of a disc) and finally sintered in air, in inert gas (nitrogen, argon, . . . ) or under vacuum at temperatures that generally range from 400 to 600° C. but which can also reach 900° C. or more before being usable in a sputtering process.
This target manufacturing process is then long and difficult to put into practice at an industrial scale. In addition, the high temperatures needed by this process lead to the evaporation or to the sublimation of some of the most volatile compounds present in the mixtures that will change the final composition of the starting material and thus the final composition of the thin film solid electrolyte that will be finally deposited onto the substrate.
Another process sometimes usable to prepare sputtering targets consists in molding the mixtures of the different pulverulent components in a mold under pressure. However, the targets thus obtained even dense are brittle and need to be further sintered to enhance their mechanical resistance. This sintering step has often to be repeated twice to obtain targets having satisfactory mechanical resistance properties.
The use of processes involving such high temperatures is a limiting factor because it forbids the presence of volatile at low temperature compounds and/or of sensitive to high temperature compounds in the solid electrolyte starting composition.